The present invention relates to a new process for the production of polyamines of the diphenylmethane series having a high diamine content and a low 2,2'-diamino-diphenylmethane content by reacting aniline with formaldehyde in two stages in the presence of hydrochloric acid as catalyst.
Polyisocyanate mixtures of the diphenylmethane series having a diisocyanate content of more than 85% have become increasingly interesting from a commercial standpoint for use in making polyurethanes, since they are only just below the high-purity diisocyanates in terms of quality, but can be produced much more economically. As far as their properties are concerned, it has often proved to be favorable for them to have an average 2,4'-diisocyanato-diphenylmethane content of from about 8 to 12% because their tendency towards crystallization is thus suppressed without any significant reduction in their reactivity. The production of the high-diamine polyamines on which these isocyanates are based is known in principle in the art. They are obtained by carrying out the condensation of aniline with formaldehyde, using a high molar ratio of aniline to formaldehyde.
A significant cost factor in the commercial manufacture of these products is the quantity of hydrochloric acid used because, normally, the hydrochloric acid must be removed from the process by the addition of sodium hydroxide.
The degree of protonation is the percentage of the amino groups present in the form of ammonium groups (i.e. amino groups neutralized with hydrochloric acid) based on the total quantity of all the amino groups (i.e. neutralized and non-neutralized amino groups in the reaction mixture). If a low degree of protonation is selected in order to obtain economically favorable consumptions of hydrochloric acid and sodium hydroxide (despite the high excess of aniline required to obtain the high diamine content), not only are increased contents of 2,4'-diamino-diphenylmethane obtained (which may often be desirable) but 2,2'-diamino-diphenylmethane is also formed to a greatly increased extent. This component leads not only to a distinct reduction in the reactivity of the polyisocyanate mixture produced from the polyamines, but it also has an adverse effect upon the properties of the polyurethane plastics produced from those polyisocyanates.
U.S. Pat. No. 3,367,969 describes a process for the production of polyamine mixtures of the diphenylmethane series comprising (i) reacting from 1.5 to 6 moles (and preferably from 2 to 5 moles) of aniline with 1 mole of formaldehyde in the presence of from 0.25 to 1.1 mole (and preferably from 0.95 to 1.05 moles) of hydrochloric acid per mole of aniline at 20.degree. to 50.degree. C.; (ii) subsequently adding at least 0.2 mole of aniline so that, in its final composition, the mixture contains from about 2 to about 10 moles (and preferably from about 2.5 to about 5 moles) of aniline per mole of formaldehyde and from about 0.1 to 0.98 mole (and preferably from 0.75 to 0.95 mole) of hydrochloric acid per mole of aniline; and (iii) heating the mixture to 50.degree.-100.degree. C. until the reaction is complete. After the acidic reaction mixture thus obtained has been neutralized, excess aniline is separated off and the 4,4'-diamino-diphenylmethane is recovered by distillation. This method of producing pure 4,4'-diaminodiphenylmethane is attended by the disadvantage that secondary products, particularly in the form of polyamines of relatively high functionality (distillation residue), accumulate in considerable quantities.
In all but one of the examples of the above-mentioned U.S. Pat., a degree of protonation of 100% (i.e., a slight excess of hydrochloric acid is used) is maintained in the first stage and a degree of protonation of at least 50% is maintained in the second stage of the reaction. Quite apart from the high consumption of hydrochloric acid and the sodium hydroxide required for its neutralization, a procedure such as this leads to a polyamine mixture from which 4,4'-diamino-diphenylmethane can admittedly be isolated in a high yield. If, however, the 4,4'-diamino-diphenylmethane is not isolated, the polyamine mixture is attended by the disadvantage that the corresponding phosgenation product (i.e., the corresponding polyisocyanate mixture) shows a pronounced tendency towards crystallization at room temperature and has a relatively high chlorine content. Finally, the corresponding polyisocyanate mixtures are dark in color.
The object of the present invention is to provide a new process for the production of polyamines of the diphenylmethane series by which it is possible to produce polyamines which contain at least 89% by weight of diamines and from 8 to 12% by weight of 2,4'-diamino-diphenylmethane content. The resultant polyamines may be used for producing the corresponding polyisocyanate mixtures which are distinguished by high reactivity, have a minimal tendency towards crystallization and which are light in color. At the same time, the new process utilizes relatively small quantities of hydrochloric acid catalyst.